Heretofore, a technique has been employed for producing aromatic carboxylic acids, such as terephthalic acid etc., in an industrial large scale, in which an alkylbenzene, such as p-xylene or the like, is subjected to a liquid phase oxidation in a solvent containing a lower aliphatic carboxylic acid, such as acetic acid, with molecular oxygen in the presence of a catalyst, such as that containing cobalt or manganese and bromine.
The aromatic carboxylic acids produced by this prior technique are contaminated with impurities of intermediates and other compounds originated from the secondary reactions, whereby the product quality of the resulting aromatic carboxylic acid, in particular, as concerns, for example, the hue of the pulverized product and the light permeability of aqueous solution obtained by dissolving it in an aqueous solution of a base, is debased considerably. For the sake of a countermeasure thereto, the reaction has hitherto been conducted by adopting severe reaction conditions by, for example, elevating the reaction temperature and increasing the catalyst concentration, under an intimate control of the reaction so as to lower the content of the impurities deteriorating the product quality.
Under such severe reaction conditions, however, a part of the lower aliphatic carboxylic acid used as the solvent will be lost during the reaction by conversion into carbon dioxide, carbon monoxide and other by-products. In general, the severer the reaction consitions, the higher will be the proportion of decomposition of the lower aliphatic carboxylic acid. Therefore, by selecting more severe reaction conditions so as to achieve a more higher quality of the product, the decomposition of the lower aliphatic carboxylic acid used as the solvent becomes greater, resulting in an increase in the running cost.
If, in contrast thereto, an oxygen-rich gas, such as pure oxygen, is employed for the molecular oxygen-containing gas, a high quality aromatic carboxylic acid can be obtained, since the occurrence of impurities badly affecting the product quality will be reduced. Here, however, the oxygen content in the gas phase in the reactor should be high enough in order to maintain a high oxygen partial pressure therein, whereby a danger of explosion of the existing easily combustible substances, such as the lower aliphatic carboxylic acid, the alkylbenzene etc., will become higher and it becomes unavoidable to limit the reactor operation condition.
In the production of an aromatic carboxylic acid by the conventional process, carbon dioxide, carbon monoxide and other by-products are discharged in accompaniment with the reactor exhaust gas. These by-products include noxious substances, such as carbon monoxide and methyl bromide, which must either be treated by any kind of purification equipment, so long as a reasonable tolerance to the environment shall be taken into account, or discharged out to the atmospheric air by an appropriate practical way in respect of the landing pollutant concentration.
The existing installations for producing an aromatic carboxylic acid, such as terephthalic acid, are usually of large scale, so that the amount of the reactor exhaust gas is also large when air is used as the molecular oxygen-containing gas. Therefore, the installations for treating the noxious substances contained in the reactor exhaust gas and for recovering by-products into useful products has to be designed also in an uneconomically vary large scale.
While the amount of the reactor exhaust gas can considerably be reduced by using pure oxygen as the molecular oxygen-containing gas, the reactor operation condition is here restricted due to the increase in the danger of explosion, as mentioned above.
Processes have hitherto been proposed for producing an aromatic carboxylic acid under recirculation of the reactor exhaust gas to the reactor (See, for example, Japanese Patent Application Kokai Nos. 36439/1985 and 83046/1988, which correspond to the U.S. Pat. Nos. 4,593,122 and 4,827,025, respectively). In the proccess of the Japanese Patent Application Kokai No. 36439/1985, however, it is difficult to attain an effect of reduction of the occurrence of impurities deteriorating the product quality by employing a high oxygen content gas having a oxygen concentration higher than that of air, since this process employs air as the molecular oxygen-containing gas to be supplied to the reactor. By the process of the Japanese Patent Application Kokai No. 83046/1988, no reduction of danger of explosion accident can be expected, when a high oxygen content gas is employed as the molecular oxygen-containing gas, since the reactor exhaust gas is returned to the gas space of the reactor.